Kyou kara Ma no Tsuku Jiyuugyou is the novel the anime and manga are based off of. It's about a boy named Yuuri, who gets himself flushed down a toilet into another world. There, he finds out he's the Maoh (demon king) and that the Mazoku (demons) expect him to lead them against the human race.

(The novel is written from Yuuri's point of view, with plenty of side-commentary that is not present in the anime)